Management of Soil-borne Plant Parasitic Nematodes for Sustainable Production of Field Grown Tomatoes and Cucumbers by Cover Cropping

1. Determine the effectiveness of selected cover crops in reducing and/or controlling plant parasitic nematodes in commercial tomato fields.

A field experiment was conducted at North Shore Farms (a commercial tomato farm) in Waialua on the island of Oahu. The experiment was repeated in 2000 and 2001. Treatments were Bare Ground Fallow (6 months fallow), soil fumigation with Metam Sodium, and two cover crops with known root knot nematode reducing characteristics: Sunn Hemp (Crotolaria juncea) and Yellow Mustard (Sinapsis alba). Experimental design was a randomized complete block with 3 replications per treatment. Plot size was double row, 60 feet long, 5 feet apart. Windbreaks of sudex grass separated every two plots. Data rows were the innermost of each plot (furthest from windbreak). All plots were prepared well in advance of tomato planting. Cover crops were seeded, allowed to grow approximately 3 feet high and then plowed in to plots for decomposition prior to planting tomatoes. Bare Ground Fallow plots were maintained dry and weed free for six months before tomatoes were planted. Metam Sodium plots were maintained in a weedy state until all plots were covered with plastic mulch for tomato production. After beds were prepared for tomatoes (soil preparation, plastic mulch, drip irrigation installed) Metam Sodium was injected into the test plots 17 days before tomato transplant. Preplant weed control was with Gramoxone Extra at labeled rates. After transplant all cropping practices (nutrition, irrigation, pest management) were the same for all treatments. Tomato cultivar used in this study was Merced, a determinate, concentrated fruit set variety. The first iteration of the trial was completed on October 31, 2000, the second on August 31, 2001. Because of low numbers of root knot nematode in 2000 all plots were inoculated with root knot nematode infested soil at the time cover crops were planted for the 2001 test (November 18, 2000) at a rate of 9 quarts of infested soil per plot. Nematode inoculated plots were irrigated immediately to maintain soil moisture necessary for root knot nematode survival.

The effect of nematodes on tomatoes was quantified by examining the effect of cover crop treatments on yield and nematode numbers. Yields for all treatments were relatively good for both runs of the trial (Table 1). Average per acre yield for tomatoes in the U.S. is 28,000 lb. (Knott’s Handbook for Vegetable Growers, 1997). A good yield is 41,000 lb. per acre. Except for the Yellow Mustard treatment all yields were greater than 30,000 lb. per acre for both trial periods. Statistically there were no significant differences between treatments with respect to yield. However, yield was consistently higher in the Sunn Hemp plots when compared with Yellow Mustard and Bare Ground Fallow (10–30% greater in Sunn Hemp). Root knot nematodes were very low in the 2000 trial but much higher in 2001 (Table 2). There were no statistically significant differences between treatments because of high variability between replicates but numbers were lowest in Sunn Hemp and Yellow Mustard plots under high nematode pressure in 2001 (Table 2). Root knot nematode pressure from gall formation was not significantly different in 2000 and 2001 (Table 3). The relationship of number of root knot nematodes to yield was significant at P = 0.10 (Table 4) for the 2001 trial. As number of root knot nematodes increased yield decreased with root knot nematodes accounting for 31.5% of the yield variation (r² = 0.315).

Reniform nematode pressure was high in 2000 and 2001 but no significant differences between treatments were detected (Table 2). The effect of reniform nematode was a reduction in root mass (Table 4). Root mass reduction was significant at P = 0.10 (Table 4) in 2001 with very high reniform nematode pressure. The reduction in root mass accounted for 25.1% of the yield variation (r² = 0.251).

The conclusion of this study is that cover cropping did not significantly influence the impact of nematodes on tomato yield. However, there was no apparent advantage to the industry standard treatment of Metam Sodium for nematode control. Additionally, there was some benefit of Sunn Hemp on production as average yields were highest with that cover crop. The benefits of cover cropping extend beyond the possibilities for nematode management and may contribute in other ways by improving soil organic matter content and serving as green manure. Additionally, the use of cover cropping reduces soil erosion problems associated with a bare ground fallow approach to nematode management and can reduce the need for soil biocide treatments such as Metam Sodium.

2. Determine the economic feasibility of using this control tactic within an existing tomato farming operation.

Results of the economic feasibility of using cover crops in tomato cropping systems in Hawaii are presented in Table 5. The benefits of Sunn Hemp as a cover crop translate into an additional $860.94 gross profit per acre. There were 420 acres of tomatoes grown in Hawaii in 1999 (National Agricultural Statistics Service, October 2000) which translates into a potential $361,594.80 increase in farm value.

3. Share the information obtained with tomato growers in the state via University of Hawaii extension personnel and publications.

Information on this project was shared with the ag community and the community at-large throughout the course of the study period by conducting field days on January 6, 2001 and June 30, 2001. Field day notification and dissemination of results was by newspaper (Honolulu Advertiser, North Shore News), radio (KCCN – The Farm Report), television (KITV 4 News), Hawaii Farm Bureau Federation, and the University of Hawaii Cooperative Extension Service. The January 6 field day focused on the cover crops and treatments. Cover crops were in the ground and participants were given handout materials detailing the SARE project and the uses of cover crops. On June 30 the field day centered on the effects of the cover crops on the tomato production. Costs of production and Integrated Pest Management were also discussed. Published reports for the SARE project are planned for the Hawaii Agricultural Research Center Annual Report (distributed to farmers and agencies statewide) and an article for the trade journal Agriculture Hawaii. Additionally, information and experience generated from this project are being leveraged to request funding for additional cover crop projects with the Hawaii Department of Health under the Environmental Protection Agency, Section 319, Clean Water Act. Grant proposal being submitted by the Hawaii Farm Bureau Federation.